Conducting a monohybrid cross of f1 generation plants, the recessive trait reappears in f2 plants in a ratio of 1: 1 2: 1 3: 1 4: 1

Conducting a monohybrid cross of F1 generation plants, the recessive trait reappears in F2 plants in a ratio of 3:1. Let allele A be dominant, and allele a be recessive. Genotypes (and their phenotypes) will be: AA (dominant trait), Aa (dominant trait), aa (recessive trait). In F1 generation, all plants are heterozygotes Aa. After they cross (Aa x Aa), the offspring will be as following: AA (dominant trait), Aa (dominant trait), Aa (dominant trait), and aa (recessive trait). So, there will be 3 plants with dominant traits and one plant with recessive trait.

Gay-lussac's law i think.